I know... I'm an idiot, but please allow me to explain (z32 brake calipers)

[cleantune]

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So, I'll get this over with.... I'm an idiot, haha. Umm, so I shouldn't have done this and every ounce of my better judgement tore the idea from my grasp, but based on the circumstances, my stupidity took advantage of a small window of time (a weekday off from work) and pushed me to make this decision.

I split a pair of Aluminum 30mm Z32 front calipers and a pair of Aluminum z32 rear calipers...I know stupid decision right?!

Despite the calipers rough appearance (the pic below makes them look better than they actually were)....


.... I had just planned on replacing the piston seals and boots. I "tickled" the temptation to split the rear calipers as I touched the rear bolts holding the two halves together. Much to my surprise two of these "bridge bolts" were only finger-tight! I was going to just pick up new used rears, but I couldn't get in touch with the person selling the calipers (I have had a difficult time trying to find just the rear calipers fs); in hindsight I should have just returned these as a "core".

So now I did this....


..Ugh, I know I'm still cringing when I see these pics; and this (now both fronts are split).


NO ONE wants to commit to a torque spec for these bolts; especially with the Aluminum version calipers; and understandably so - The FSM says: ah no no dont tatch deese and if it had hands it would reach out and slap the **** out of the wrist of anyone holding a wrench to them .

Since the FSM has no number for these, guessing seems to be the "internet guide's" method-of-choice for this(I have found a huge range anywhere from 60ft-lbs to 14 ft-lbs w/ red loctite). To be fair though, without being able to test 100's of these calipers and bolts to their "breaking points", I suppose their torque spec is a guess; just aiming for an educated guess with a little more proof than "I heard this guy did __ft-lbs and they work fine".
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Rather than just ask what to torque these M10x1.25 "bridge bolts" to, I thought this may be a (or another) good place to discuss unknown torque specs for bolts.

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[cleantune]

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So far I've found the following info; please feel free to correct me and/or add to this:
1. "break-away torque", I think its called, (where one gradually increases torque on wrench until the bolt breaks free) is NOT a valid way to determine torque values.
- oxidation may have formed on the threads, affecting its thread "grip"
- random burs or defects in the threading may be present in that particular
bolt, but not on other bolts of the same dimensions
- unknown lubricant and/or thread locker may have been used on the bolt (a
finger tight bolt with a thread locker compound on it usually takes more
force to remove than what it was put on with).

2. I've read that generally the torque spec is based ONLY on the bolt (since this rating is usually based on how much "stretch" the bolt is required to have to seat the threads tight inside one another); however, I would like to argue or just explain that I think the material it's threaded into would have to matter as well- for instance carbon steel bolt threading into a brass threaded plate would take less torque to secure than a carbon steel into carbon steel assembly.

3. Lubrication generally lowers the "dry" torque spec given for a bolt One website stated about 15% lower torque spec when lubrication is used on the threads.

4. Dry torque specs are also generally lower if the bolts are plated in something for corrosion resistance. For example: zinc or nickle plated hardware. This kind of makes sense, since material is being added to the outside of the threads (even if it is a micro-amount); which would make the gap between threads smaller to begin with, therefore the bolt would need to stretch less, so less force would be needed.

-Further more, if calculating an estimated torque value based on the bolt specs, each type of plating has its own friction coefficient (in addition to the friction coefficient of the material it is threaded into.

** I know this topic may be a bit "engineer-rich", but with something like brakes (or other important safety parts), I think the extra attention to the physical details can be justified.

Links:
general info:

http://fp.optics.arizona.edu/optomech/Fall14/Notes/18%20Fasteners.pdf

Torque chart (w/o thread pitch):

https://www.swtc.edu/ag_power/diesel_engines/lecture/metric%20torque%20values.pdf

Torque chart (w/thread pitch):

http://www.engineershandbook.com/Tables/torque1.htm

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[Matt93SE]

the concern with high torque is the fine thread bolts going into cast aluminum. I have a set of them that the threads were stripped because they were over torqued, and then they came apart while on the car. he called and asked why he had to keep adding brake fluid. As soon as he pulled the wheels off, he knew the answer. eek.

anyway, be wary of overtorquing them for that reason. I'd rather see a bit less torque and loctite.

Be sure to replace the o-rings too. you can buy replacement square o-rings at McMaster.

[gills]

Instead of regurgitating too much, here's direct scans from Screws, Fasteners, and the Design of Nonpermanent Joints section from Shigley's Mechanical Engineering Design textbook (8th edition) used in many University level Mech Engineer programs. May help you establish a torque to tighten to, although threading into softer materials like aluminum isn't covered. Just a quick blurb about being "very careful not to use a soft material in a threaded fastener" at the end. Bolt condition/surface treatments are briefly covered and how they affect the bolt torque and tension. A good read if you're into this kind of stuff.

[Matt93SE]

something else to keep in mind.. tightening torque for VQ35DE transmission to engine bolts is 55ft lb. These are 10mm coarse thread bolts threading into aluminum and it's my understanding that fine thread in aluminum needs to be lower torque because there is less cross sectional area at the root of the thread to resist pullout.

I also did some quick googling and found this:

http://www.futek.com/boltcalc.aspx

There's a small link at the top that allows you to switch to metric values. enter 10x1.25mm with a 15mm engagement, and the results come out to 35-46 ft lb as the proper specs.

I would shoot for 40 ft lb with red loctite and call it a day.

[djsilver]

You've already been given some good information here.
-I would like to clarify that you will strip the threads in the aluminum long before you get any stretch on the bolt.
-The advantage of a fine-thread bolt is that it can apply more clamping force than a coarse thread with less torque.
-If there's evidence that loctite type stuff was used originally, you should carefully use a chaser tap to clean the material out of the threads. You only want to clean out foreign material and not enlarge the diameter of the threads.
-Before you apply loctite for the final assembly, I would recommend a dry fit, to determine how much torque is required to flatten the o-rings. This will eliminate confusion or questions during the final assembly.

[gills]

Are there definitely not steel helicoil type inserts in those holes? Because the holes to mount the calipers to the knuckles do have inserts on my aluminum Z32s. I remember being surprised about that because the brembos on my STi didn't and those threads were destroyed the first time I removed the bolts. They were over-torqued from the factory. Very common problem on that car.

[gills]

Here's a Wilwood PDF that shows a torque spec for 7/16-20 (fine thread) bridge bolt at 47 lbs-ft with red loctite for a Superlite caliper, which are aluminum, correct?

http://www.wilwood.com/pdf/datasheets/ds221.pdf

I'd say that provides a pretty good idea of where you should be.

[Matt93SE]

Are there definitely not steel helicoil type inserts in those holes? Because the holes to mount the calipers to the knuckles do have inserts on my aluminum Z32s. I remember being surprised about that because the brembos on my STi didn't and those threads were destroyed the first time I removed the bolts. They were over-torqued from the factory. Very common problem on that car.

Nope, they're plain aluminium.

[Def]

Keep in mind loctite provides similar lubrication to 10W-30 engine oil. This will definitely lower torque values.

Without doing any calcs, I think somewhere in the 32-38 ft-lb range is reasonable on an M10 fastener going into aluminum.


Fine threads in aluminum are not easier to pull out, as the thread root is what provides all the strength. They are less resistant to damage, and the fastener itself is stronger, so it becomes a bit of a strength mismatch where the bolt is stronger than the threaded joint. A coarse thread fastener starts getting them roughly equivalent for a "high strength" class 10.9 type fastener, assuming no fatigue damage on the threads.

[cleantune]

Matt-

That’s the type of situation I’ve been envisioning over and over, super scary stuff. It does seem like less torque for something like a steel bolt threading into aluminum (with loctite or another suitable threadlocker) would be the “safer option, just because looser wouldn’t destroy the threads like overtorquing would.
McMaster is awesome, they have all the good stuff. Z1 Motorsports also sells these now with “their” rebuild kits or they can be bought separately from them (front and rears w/ square edges); hah, I wouldn’t be surprised if they got them from McMaster Carr.
Thanks for finding that “FUTEK link too; another great reference tool.

Gills-

Thanks for saving all of us from a lecture haha; and for including those scanned pages. Even though the softer material apps are not included in full detail, I’m sure the same principles apply to some extent. Best believe I am planning on reading through ALL of this and taking notes like I’m trying to get that “A” on a test. I’ll be adding any “key” info I find here that has not alreadybeen included.

Djdilver-

Tons of great info so far, thanks for clarifying any question about the “bolt stretching". I think you’re right- this bolt stretch probably only applies if the bolt and thread material are the same or if the threaded material is stronger than the bolt; just by following the physical characteristics of materials used in the assembly. I did not notice any loctite or other thread locker remnants on the threads, just a little bit of chalky oxidation where some of the zinc plating had been rubbed off during assembly. I’m half-wondering if the Aluminum threading is what “stretches” to close any air gap between the threads. That’s a good idea to dry fit everything first.

[cleantune]

That Wilwood link is great. This could be used as a “check” too for torque spec answers found from any math or online info.

I just checked the “bridge bolts” thread again and they are all aluminum. I held a magnet to them too, since the threads are accessible from the front side of the caliper; not ferrous metal. The threads also seem to “spill out” to the outside edge of the caliper; meaning that the threads are not squared off at the end; instead, they match the contour of the outside edge of the caliper as if they were cut into the caliper (if that makes sense). There is also no visible boundary between the threads and the aluminum caliper body.

I have been checking out similar applications for their torque specs, which is actually where I heard the 14 and loctite. Someone locally did a GT-four brake swap and these brakes are almost identical to the z32 brakes- 4piston front, 2piston rear, aluminum and even made by Sumitomo!. They had gotten their specs from a similar RX7 caliper (I think these are Sumitomo as well), but again, just “hear-say”, without proof other than "they are together".

I'm also wondering if there is a "rule" about multiple installs with hard metal bolts into softer metal. If the harder bolt "shapes" or stretches the softer metal threads, maybe the second install requires less torque with thread locker? I dunno, I've got a lot of reading to do.

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[djsilver]

I feel bad for you that you're stressing so much over this. I guess I broke enough hardware when I was younger that it eventually "calibrated me"! I haven't wrung off any fasteners or stripped any threaded holes in years..., The only thing I've had fall off a moving car was a tape measure I left under the hood. And yes, I know I just jinxed myself. ;-)

[cleantune]

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aww nah, don't feel bad; overdoing things is my specialty ;] If there was a two piece puzzle with more than one way to assemble the pieces, best believe that I would have a "laundry list" of pros and cons for all the outcomes, haha.

Somewhat similar in nature, like your experience with various fasteners, this "research" is more just to build confidence in whatever guess/ choice will be made for the torque specs.

Gills, late last night I started looking over those pages you had posted and was taking notes on it. After I get through it all, I'll try to condense the info into a "bite-sized chunk" of application-specific info...if possible, haha.

Not that it matters now, but does anyone know if there is any merit in counting number if turns w/ a reference point?
For example the steel bolt is fastened in an all aluminum threaded block. One draws a line off of the one side of the bolt head and onto the block, then uses this line as a reference point to count how many turns it takes for the bolt to be free of the aluminum threads. Could this be a valid way to reassemble a fastener "tight-enough"?

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[Def]

Once you reach approximately 5-6 full threads, you've reached "max strength" of the bolted joint. If you want to give the softer metal extra strength in the equation, roughly 8-9 threads is definitely where you are at full strength in a joint.